The present invention is related to a lifting vessel having a U-shaped pontoon, with two longitudinal and one transversal pontoon, and columns connected to the pontoon extending upwards and through the water surface.
The present lifting vessel is designed with a hull where the buoyancy can be adjusted by ballasting/deballasting, for lifting and transportation operations at sea.
The invention includes also methods for positioning, lifting and handling a platform deck and a jacket, with the help of the lifting vessel of the present invention.
In connection with offshore activities such as gas and oil exploitation it is usual to install platforms on the field. These platforms often consist of large and heavy platform substructures fixed to the seabed. Such a platform substructure is normally a so-called xe2x80x9cjacketxe2x80x9d, which is a steel truss structure. On top of for example a jacket it is usual to place a platform deck, which is used in connection with drilling and production. The deck also often includes living quarters.
To transport and install the jacket and the platform deck described above, for example barges have been used to transport the jacket and platform deck out to the field, and large crane vessels have been used to install the platform on the field.
Heavy lift vessels using ballast to vary their draft have also been used to transport and install platforms offshore.
There are today a great number of offshore platforms installed to exploit oil and gas. When the oil and/or gas reservoirs are fully exploited the life span of the platform is usually over and it would in most cases be appropriate to remove the platform.
Some platforms are already removed, and removal of platforms will continue at an increasing pace the coming years.
The traditional way of removing platforms is to use large ocean going lifting cranes. The platform needs to be very thoroughly prepared prior to removal, and it must be cut into smaller parts since even the largest lifting crane vessels have limited lifting capacity. The same goes for the platform substructure (the jacket).
These operations are time consuming and costly, not only because the lifting cranes are large, expensive and need a large crew, but also because cutting a platform to smaller pieces in open sea is a very complicated task. It is also a risky operation.
The new technology, as described in this application, can be described as xe2x80x9csingle lift technologyxe2x80x9d, and will reduce the costs considerably. It will also make the operations less risky than present alternatives. Within the category xe2x80x9csingle lift technologyxe2x80x9d there are three other concepts that the applicant is aware of at the moment:
xe2x80x9cOffshore Shuttlexe2x80x9d is a vessel planned built as a frame work structure. The vessel has a significant length and the lifting of for example a platform deck is based on crossbeams spanning across the structure.
xe2x80x9cMaster Marinexe2x80x9d is developing a U-shaped semi submersible with deck-structure connecting the top of columns. Lifting is based on load transfer to the deck-structure.
xe2x80x9cVersatrussxe2x80x9d is a concept involving two separate barges each supporting its own lifting frame. By pulling the barges together after positioning the lifting frames beneath the lifting points on the platform deck, the lifting of the deck can be performed. This method has already been used to remove small platform decks in calm waters.
One object of the present invention is to accomplish a removal operation of a platform in a fast and cost effective manner without cutting either the deck or the jacket into smaller parts. The removal operation shall be performed in a safe way where the safety of the operators is accomplished in the best possible way.
Another object of the present invention is that the lifting vessel is as flexible as possible and that it can be easily adjusted to fit different sized platform decks. Further the lifting vessel shall be able to lift and handle jackets of different sizes. In accordance with the invention the lifting vessel, a so-called Multi Purpose Unit, MPU, which also can transport e.g. the platform deck to shore, and then transfer the deck to a barge or a pier suitable to the vessel.
Another object is that the lifting vessel shall be able to be transported on a heavy lift vessel, to reduce the travel time to distant destinations like the Gulf of Mexico and the Persian Gulf.
Another object of the lifting vessel is that it shall also be able to install platforms, which basically is the reverse of removal. The lifting vessel should furthermore be applicable to a range of purposes where a large lifting capacity is required.
The objects described above is achieved according to the invention by lifting vessel with a U-shaped pontoon, a number of columns connected to the pontoon and extending upwards towards and through the water surface, characterised by the columns not being structurally connected above the pontoon.
Preferred embodiments of the lifting vessel is further described in the claims 2 to 12.
The objects of the invention is further achieved by methods of positioning, lifting and handling of a platform deck and a jacket according to claims 13 and 14.